Torque control system for a gas turbine

ABSTRACT

A gas turbine control system has a plurality of closed loop controls, each depending on a different operating parameter and providing a fuel control signal in accordance therewith. A predetermined linear relationship exists among the power output and the output shaft speed at the upper limit of acceptable output torque which is developed. A comparator is employed to compare the predetermined power setting signal applied to the gas turbine and a signal representative of the output shaft speed. Through this comparison it is possible to limit the output torque developed in the transmission components to an acceptable maximum value.

ijnited States Patent 1191 Rowen TORQUE CONTROL SYSTEM FOR A GAS TURBINEPrimary ExaminerCarlton R. Croyle I Assistant Examiner-Robert E. Garrett[75] lnvemor' g Rowen Schenectady Attorney-William C. Crutcher, James W.Mitchell, l V Frank L. Neuhauser, Oscar B. Waddell and Joseph B. [73]Assignee: General Electric Company, Schenec- Forman tady, NY. 22 Filed:Mar.26, 1971 [57] ABSTRACT A gas turbine control system has a pluralityof closed [21] Appl' l28399 loop controls, each depending on a differentoperating 52 use]. ..'.60/39.2s P, 416/30 60/39.l7 Parameter andPmviding a fuel Signal in 51 Int. Cl. r026 9/08 Cordance therewith Apredetermined near 58 Field Of Search ..60/39, 28 R, 39.28 P, Shipexists the and the 60/3916 R, 17; 415/17 shaft speed at the upper limitof acceptable output torque which is developed. A comparator is employed[5 6] References Cited to compare the predetermined power setting signalapplied to the gas turbine and a signal representative of UNITED STATESPATENTS the output shaft speed. Through this comparison it is 3,606,7549/1971 White ..60/39.l4X possible to limit the Output torque developedin the 3,421,317 H1969 Bedford 60/39.28R transmission components to anacceptable maximum 3,520,133 7/1970 Loft et al. 60/3928 R value,3,106,062 10/1963 Rosenberg et al. ..60/39.28 R 3,255,586 6/1966 Henniget a] ..60/39.28 R 2 Claims, 2 Drawing Figures meat. 16 a CONTROL FUELCONTROL START-UP l 23 CONTROL MODE {SELECTOR SINGLE l9 1 49 4g} uit t'iifi w uuum BRIDGE PROPULSIW LEVER I (ASTERN) 1:

ENGINE ROOM PROPULSION LEVER Patented May 1, 1973 3,729,928

2 Sheets-Sheet l NOZZLE CONTROL I w i i l2 n J l I l5 V 8 PD [OJCOMPRESSOR I f 2 TEMP AVE I, 5/ i TEMR j CONTROL SPEED} 7 :e I I LOAD 2|CONTROL 1 AccEL. l8 22 CONTROL' I 7 FUEL i CONTROL I START-UP I8 CONTROLMODE F SELECTOR SNGLE I .QUADRANT mm f MULTIPLIER t -(AHEAD) BRIDGEPROPULSION (ASTERN) (AHEAD) ENGINE ROOM PROPULSION LEVER 9 (I w=sk xINVENTOR WILLIAM I. ROWEN Patented May 1, 1973 POWER TORQUE LIMITINGSPEED FUNCTION STEADY-STATE POWER REQUIRED 2o 40 so 90 I00 PROPELLERSPEED IINVENTOR TORQUE CONTROL SYSTEM FOR A GAS TURBINE BACKGROUND OFTHE INVENTION The present invention relates generally to an improvedcontrol system for a gas turbine power plant I bine has become quitecomplex, but as a result of the increasing complexity, the outputcharacteristics have been enhanced together with an increased lifetimefor many of the component parts. Such controls normally incorporatemeans to influence the rate of fuel flow to the gas turbine combustionchamber in accordance with fuel control signals obtained by monitoringthe operating parameters of the gas turbine. Examples of such controlsystems are known in the art and may be found in US. Pat. No. 3,520,133issued July. 14, 1970, to A. Loft et al. for a Gas Turbine ControlSystem" and US. Pat. No. 3,639,076, issued toW.I. Rowen on Feb. 1, 1972,for a Constant Power Control System for a Gas Turbine, both assignedtothe assignee of the present invention.

The subject matter of the present invention is directed to an improvedtorque limiting featureincorporated into a power control loop. Prior tothe subject invention, a predetermined power setting was applied to apower control loop for a turbine to condition the apparatus for aspecified fuel flow or power output. The predetermined power setting wasdependent, how ever, on the other operating parameters of the turbinewhich parameters could be calling for a fuel flow below thatcommensurate with the, predetermined power setting. When thepredetermined power setting was not limited, however, by one of theother operating parameters, there would be a call for a fuel flowcommensurate with that of the predetermined power setting. The output ofthe power control loop would then be compared tothe actual fuel flowthrougha representative feedback signal.

The subject invention is concerned with a method of limiting an appliedpredetermined power setting when the output shaft is subjectedtoconstant propellerpitch operation and conditions which cause a reductionin output shaft speed. If for some reason, the output shaft should beoverloaded, the inducedtorque would necessarily increase at anyoperating predeten'nined. power setting. Further, the subject inventionis. concerned with similarly limiting an increased power setting whichhas been appliedjto the gas turbine, resulting in increased power to theoutput shaft. If, because of load on the propeller for example, thespeed of the output shaft does not increase as fast as the increase inpower, overtorquing can occur.

In the present embodiment, three modes of operation of the gas turbinepower plant are envisioned. In the first, a minimum power settingissupplied to the power control loop and is independent of a propulsionlever position. In this arrangement the ship's speed is varied by a zeroto full setting on the propulsion lever position effecting, through afirst pitchcontrol, a corresponding zero to full propeller. pitch statuson the gas turbine output shaft.

In the second mode of operation, the propulsion lever position effectsan input to a first pitch control loop, utilized in the first mode ofoperation, and also effects a predetermined power setting input into thepower control loop which is added to an already existing minimum powersetting. In this mode, full propeller pitch is achieved in approximatelythe first 25 percent travel of the propulsion lever while the vesselspeed response is designed to linearly increase throughout the entirepropulsion lever range.

In a third mode of operation, a desired power setting, indicated by thepropulsion lever position, induces a corresponding fuel flow to thecombustion chambers of the gas turbine. In this arrangement a secondpitch control loop is utilized while the first pitch control isinoperative. An output shaft speed signal is utilized as an input to thesecond pitch control. In this mode it is desired to maintain the outputshaft speed constant for a predetermined power setting by varying pitchaccordingly. Therefore, the output shaft speed signal indicates tothesecond pitch control any variation in the speed of the output shaft.

Accordingly, it is an object of the present invention to limit theamount of torque which can be induced the second and third modes ofoperation when the ship is subjected to conditions causing either adecrease in the output shaft speed or an increase in the power setting.

SUMMARY OF THE INVENTION Simply stated, the invention is practiced intwo modes of operation by comparing; the predetermined power settingsignal applied to the gas turbine and a signal proportional to the speedof the output shaft. A gating device used for this comparison, passingthe lower of the twosignals, thereby limits the amount of torque whichcan be induced in the transmission components of the system. Apredetermined linear relationship exists between the power output andthe output shaft speed at the upper limit of acceptable output torque.

DESCRIPTION OF THE DRAWINGS The subject matter which is regarded as theinvention is particularly pointed out and distinctly claimed in theconcluding portion of the specification. The invention, however, both asto organization and method of practice, may best be understood byreference to the following description, taken in connection withtheaccompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of a DC. analog controlapplicable to a gas turbine for carrying out the object of theinvention;

FIG. 2 illustrates the torque limiting aspects of the invention inrelation to the power applied by the gas turbine and the resultantpropeller speed.

DESCRIPTION OF THE INVENTION Referring to FIG. I of the drawing, asimplified representation of a two-shaft gas turbine, indicatedgenerally as 1, includes a compressor 2, combustion chamber 3, highpressure turbine 4, and low pressure turbine 5. It is known in the artthat in certain gas turbine applications, it is desirable to have thehigh pressure turbine 4 which drives the compressor 2, and the loadturbine 5 mounted on separate shafts. In the application shown in FIG.I, a load turbine shaft 6 is connected through a set of reduction gears,symbolically shown at 7, to a propeller 8. Propeller Sis of the con-.trollable'reversible pitch variety, the function of which will be morefully described later.

A variable area second stage nozzle 9 aerodynamically couples the highpressure turbine 4 with the load turbineS. The details of this nozzleare not material to the present invention and such nozzles are wellknown in the art; an example of a suitable nozzle with its attendingcontrol system is shown in US. Pat. No. 2,625,789, issued on Jan. 20,1953 to NE. Starkey and assigned to the assignee of the presentinvention.

Air entering the compressor inlet at 11 supports the combustion of fuelinjected by a nozzle 12 in the combustion chamber 3. The heated exhaustgases, after passing through the high pressure turbine 4 and lowpressure turbine 5, exit from the turbine outlet 13 past distributedtemperature sensors, such as 14 for measuring exhaust temperature. Avariable delivery fuel pump 15 is driven from a compressor high pressureturbine shaft 10 and delivers fuel to nozzle 12 at a rate of flow whichdepends both upon rotational shaft speed of shaft 10 and the pump strokewhich is set by a fuel control servo 16. The details of fuel controlservo 16 are not material to the present invention and it may includeany servo mechanism which positions the stroke setting control on thepump at a position corresponding to an electrical positioning signal,there being many devices on the market suitable for such use. Further,since the gas turbine may utilize gaseous fuel, in which case a gasvalve system would control fuel flow to the combustion chamber, the termfuel control signal is used to designate the signal regardless of thetype of fuel used.

The fuel control signal is a single valued electrical signal whichappears on line 17 and is applied to fuel control 16 by a low value gatecomprised ofa plurality of diodes 18. Diodes 18 are poled as shown withrespect to a common bus 19, from which the fuel control signal isobtained on line 17. As is explained in U.S. Pat. No. 3,520,133 Loft etal., this arrangement monitors a series of individual fuel controlsignals emanating from separate closed loop controls which arerespective to respective operating parameters of the gas turbine. Thelowest of these fuel control signals is passed on line 17 andsubsequently determines the fuel flow to the combustion chamber of thegas turbine.

A plurality of loop control means 20, 21, 22 and 23 are employed. Thecontrol 20 operates in response to an average temperature input relatedto the turbine exhaust gas area. The control 21 operates as a speed andload monitor while controls 22 and 23 respectively monitor accelerationand start-up conditions of the gas turbine. The particular details ofthe loop control means 20, 21, 22 and 23 are not necessary for thedescription of the present invention. The operation of such loop controlmeans is described in the aforesaid Loft et al. patent.

To produce an additional fuel control signal, which appears on line 31,an additional loop control means 24 is provided. The additional loopcontrol means 24 is responsive to a power set point and a feedbacksignal representative of actual fuel flow.

Generally the ship incorporating the turbine referred to herein issubject to three modes of operation, namely Docking, Maneuvering and SeaModes as selected by mode selector 49. The three modes are indicated bythe symbols D,M and S respectively in FIG. 1. In all three modes percenttravel of the propulsion levers 34 and 35 is permitted. Either lever maybe utilized to control the vessels power plant from either the bridge orengine room areas respectively.

In the Docking (D) mode the subject invention does not apply. As fullydescribed in the aforesaid Rowen application, a predetermined minimumpower signal is fed into the power control amplifier 40 through the line45. A feedback signal representative of the actual compressor speed,multiplied by a compensating gain, is also fed into the power controlamplifier 40 through line 41. Thus, the additional loop control means isconverted to a simple governor for the high pressure turbine andcompressor. During this mode of operation, with the power in the gasturbine being fixed at a minimum level, the ship speed is controlled byvarying the propeller pitch through a first pitch control generallyindicated as operational amplifier 56. As the propulsion lever isadvanced, an input to operational amplifier 56 is supplied. Operationalamplifier 56 produces an input to operational amplifier 53 through line52 which in turn controls the propeller pitch through a servo mechanism54. In this manner, a zero to full propeller pitch corresponds to a zeroto full travel position of the propulsion lever thereby effecting asensitive control of the ships speed as would be necessary for a dockingsituation. Further, in the absence of any signal through operationalamplifier 56 and line 52, zero pitch is called for on the shipspropeller.

The control system of the subject invention is of importance in theManeuvering (M) mode. When the propulsion lever has not been advanced, apredetermined minimum power signal is fed into the power controlamplifier 40 through line 45 in the same manner as in the Docking mode.As the propulsion lever is advanced a predetermined power setting isapplied to a cubic function generator 39. The function generator 39provides a first signal representative of the predetermined powersetting to the gas turbine. The function generator 39 provides the firstsignal in a manner such that the ship's actual speed will linearlyincrease with the advancement of the propulsion lever. This results fromthe fact that the ships speed is proportional to the cube of the shipspower. Therefore, as the propulsion lever is advanced calling for acorresponding linear increase in the speed of the ship, the first signalcalls for the power necessary to drive the ship at the desired speed asindicated by the propulsion lever position. Further, in this mode, thesame pitch control is utilized as in the Docking mode; however, fullpropeller pitch is reached when the propulsion lever has advancedapproximately 25 percent of its full travel range.

in the Maneuvering mode of operation in prior art systems it waspossible to encounter conditions which resulted in overtorque andtherefore in danger of damage to elements of the system. For example, ifa change in load caused the output shaft to become overloaded and thepower setting and propeller pitch setting remained the same, substantialovertorquing could occur. In accordance with the present invention agating means 79is provided for reducing the fuel flow on a temporarybasis in response to an indication of potential overtorque, until thecondition which could have caused the overtorque has terminated.

In the gating means 79 the first signal representative of thepredetermined power setting appears on line 76. A monitoring signalrepresentative of the load speed or propeller speed appears on line 78.The signal appearing on line 78 is supplied to operational amplifier 80which in turn provides a second signal proportional to the load speed orpropeller speed which appears on line 77 and indicative of the maximumallowable power which can be called for at the monitored load speed. Thegate 79 is a low value gate comprised of the diodes indicated as 81. Thediodes 81 are poled as shown with respect to a common bus 82, from whicha third signal, representative of the lower of the signals appearing onlines 76 and 77, is supplied to lines 83 and 75.

In actual operation of the gas turbine in the Maneuvering mode, if thefirst signal provided by the function generator 39 were fed directlyinto the power control amplifier 40, it would be possible to overtorquethe mechanical transmission components of the system should the outputshaft be overloaded for any reason and the propeller pitch be fixed as aconstant. The purpose of the low value gate 79 is to compare the secondsignal, specifically, a maximum allowable power signal, on line 77 withthe first signal (representative of the predetermined power setting) online 76. When the second signal on line 77 is compared with the firstsignal on line 76, it can be determined whether the operating outputtorque will exceed an acceptable upper limit. The third signal,appearing on lines 83 and 75, is added to the existing minimumpredetermined power signal and compared to the actual fuel flowrepresented on line 41. The operational amplifier 40 subsequentlyprovides an additional fuel control signal resulting from the comparisonbetween the summation of the signals on line 75 and 45 and the feedbacksignal appearing on line 41.

As seen in FIG. 2 of the drawings, the upper limit of acceptable torqueis applicable to the entire range of applied power and resultantpropeller speeds. This results from the fact that the gas turbine outputpower is equal to the product of its load speed and output torque.Therefore, when the second signal on line 77 becomes less than the firstsignal on line 76, indicating that the maximum allowable power andnecessarily the maximum allowable output torque would be exceeded if thepredetermined power setting called for by the first signal is giveneffect, the low value gate 79 passes the lower, that is second, signalonto line 83 and the first signal is overridden until the load speedcomes up to value allowing a further increase in the predetermined powersetting to the gas turbine which will not cause the upper limit ofacceptable torque to be exceeded. In FIG. 2 the above results areillustrated. A steady state operation point is shown at a. As thepropeller speed decreases due to overloading at the constant-power levela, point b is approached at which the upper limit of acceptable outputtorque is reached. Any further decrease in propeller speed causes thesecond signal to be lower than the first signal as described above andthe operating point moves along the torque limiting line to c. As thepropeller speed returns to steady state, the above path is retraced. Itwill be obvious to those skilled in the art that the above path andsteady state power curve represent boundaties between which thetransition from c to a may vary from straight line movements.

In a second situation shown in FIG. 2, an applied power setting level dis suddenly increased to level a. If the propeller speed does notincrease as fast as the power developed by the gas turbine, it would bepossible to exceed the upper limit of acceptable torque. As an example,if increased turbine power is applied at a propeller speed indicated atd, point c would be approached at which the upper limit. of acceptabletorque is reached. At point c the second signal becomes lower than thefirst signal thus preventing an increase in power which would result inthe upper limit of acceptable torque being exceeded. As the propellerspeed increases, the operating point moves along the torque limitingline to b which is at the same power level as point a. As the propellerspeed continues to increase, the operating point moves along the linebetween b and a. It will be obvious to those skilled in the art that theabove path and steady state power curve represent boundaries betweenwhich the transition from d to a may vary from straight line movements.

In the Sea mode of operation, the torque limiting aspect of gate 79 isnot necessary, although it is available. The predetermined power signaldeveloped by the function generator 39 is combined with thepredetermined minimum power signal appearing on line 45 and theresultant summation is compared with the feedback signal appearing online 41 which is representative of the actual fuel flow. In this thirdmode, a second pitch control is activated by a signal transmittedthrough lines 73 and 52. This second pitch control is in itself a torquelimiting device. A signal representative of the load speed is fed intooperational amplifier 65. The subsequent signal provided on lines 73 and52 functions to maintain the load speed constant, at any predeterminedpower setting, by varying the pitch of the propeller through servomechanism 54 in response to the signal applied to operational amplifier53. Thus, in the Sea Mode, the gating means 79 functions as a torquelimiting device in the event that the second pitch control becomesinoperative.

The additional fuel control signal supplied by operational amplifier 40appears on line 31. When the additional fuel control signal is thelowest of all the several fuel control signals, it is passed by the lowvalue gating means, represented by diodes 18, onto line 17 as a fourthsignal. The fourth signal in turn passes through the fuel control 16 tothe fuel pump 15. Thus, when conditions are present which couldovertorque the output components, the fourth signal reduces the fuelflow to the combustion chamber of the gas turbine thereby precluding anyovertorque from being developed.

While there is shown one embodiment of the invention, it is of courseunderstood that various modifications may be made therein, and it isintended to cover in the appended claims all such modifications as fallwithin the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a control system for a gas turbine driving a load at a given speedincluding a fuel supply means and servo means to control fuel flowthrough said fuel supply means to a combustion chamber of the gasturbine in accordance with a lowest value gated fuel control signaldeveloped by a plurality of loop control means, each continuouslyresponsive to a different operating parameter of the gas turbine andeach arranged to supply a respective fuel control signal to a low valuegating means; the combination comprising:

means for providing a predetermined power setting signal;

means for developing a maximum allowable power signal proportional tothe load speed; and

an additional loop control means supplying an additional fuel signal tosaid low value gating means, said additional fuel control signal beingrepresentative of the lower of a comparison of said predetermined powersetting signal and said maximum allowable power signal, whereby loadshaft torque is limited by said additional fuel control signal when itis dependent on the load speed.

2. A control system for a gas turbine, wherein the gas turbine includesservo means for controlling fuel flow to a combustion chamber of the gasturbine in accordance with a lowest value gated fuel control signaldeveloped by a plurality of loop control means, each responsive to adifferent operating parameter of the gas turbine and each arranged tosupply a respective fuel control signal to a low value gating means;comprising: an additional loop control means for supplying an additionalfuel control signal to said low value gating means, said additional loopcontrol means comprising: means for providing a minimum power signal;means for providing a first signal representative of a predeterminedpower setting; means for developing a second signal proportional to thevalue of the load speed and indicative of maximum allowable power; meansfor developing a feedback signal indicative of actual fuel flow; secondgating means for comparing said first signal and said second signal,said second gating means providing a third signal representative of thelower of said first and second signal; and summing means for combiningsaid minimum power signal, said third signal and said feedback signal toprovide said additional fuel control signal.

1. In a control system for a gas turbine driving a load at a given speedincluding a fuel supply means and servo means to control fuel flowthrough said fuel supply means to a combustion chamber of the gasturbine in accordance with a lowest value gated fuel control signaldeveloped by a plurality of loop control means, each continuouslyresponsive to a different operating parameter of the gas turbine andeach arranged to supply a respective fuel control signal to a low valuegating means; the combination comprising: means for providing apredetermined power setting signal; means for developing a maximumallowable power signal proportional to the load speed; and an additionalloop control means supplying an additional fuel signal to said low valuegating means, said additional fuel control signal being representativeof the lower of a comparison of said predetermined power setting signaland said maximum allowable power signal, whereby load shaft torque islimited by said additional fuel control signal when it is dependent onthe load speed.
 2. A control system for a gas turbine, wherein the gasturbine includes servo means for controlling fuel flow to a combustionchamber of the gas turbine in accordance with a lowest value gated fuelcontrol signal developed by a plurality of loop control means, eachresponsive to a different operating parameter of the gas turbine andeach arranged to supply a respective fuel control signal to a low valuegating means; comprising: an additional loop control means for supplyingan additional fuel control signal to said low value gating means, saidadditional loop control means comprising: means for providing a minimumpower signal; means for providing a first signal representative of apredetermined power setting; means for developing a second signalproportional to the value of the load speed and indicative of maximumallowable power; means for developing a feedback signal indicative ofactual fuel flow; second gating means for comparing said first signaland said second signal, said second gating means providing a thirdsignal representative of the lower of said first and second signal; andsumming means for combining said minimum power signal, said third signaland said feedback signal to provide said additional fuel control signal.